This invention relates to improvements in a fuel vapor treatment system, and more particularly to the fuel vapor treatment system for separating and concentrating fuel vapor generated in a fuel tank of an internal combustion engine and then liquefying and recovering fuel.
Hitherto a variety of fuel vapor treatment systems have been put into practical use for the purpose of preventing emission of fuel vapor generated in a fuel tank of an automotive vehicle to atmospheric air. A typical one of the fuel vapor treatment systems is arranged as follows: Fuel vapor generated in the fuel tank upon temperature rise in the fuel tank is temporarily stored or adsorbed in activated carbon or the like in a canister via a line connecting the fuel tank and the canister. Fuel vapor stored in the activated carbon is introduced into an intake air passageway of the engine via a line provided with a vacuum control valve under the influence of intake vacuum in the intake air passageway in such a manner that the amount of adsorbed fuel vapor does not exceed the fuel adsorbing capacity of activated carbon.
In such a conventional fuel vapor treatment system, the amount of a mixture gas containing fuel vapor to be introduced into the intake air passageway is controlled by the vacuum control valve. However, the mixture gas contains fuel vapor and air which are not accurately metered, and therefore disturbs a set air-fuel ratio of air-fuel mixture to be supplied to the engine when the mixture gas is added to the air-fuel mixture at the upstream side of a fuel injector valve through which an accurately metered fuel is injected to be mixed with intake air. This makes it impossible to accomplish combustion at the set air-fuel ratio, thereby lowering drivability of engine while affecting components of exhaust gas emitted from the engine. Additionally, it has been required to lower fuel consumption in order to deal with recent environmental problems and promote resource-saving trends. Under such requirements, the above-discussed problems encountered in the conventional fuel vapor treatment system will be more apparent in case that combustion at ultra-lean air-fuel (air/fuel) ratio (approximately 40 to 50) of air-fuel mixture is carried out by direct fuel injection to a combustion chamber rather than conventional combustion at lean air-fuel ratio (approximately 20).
In order to solve such problems, it has been proposed in Japanese Patent Provisional Publications Nos. 10-274106 and 11-93784 to provide a membrane separation module which includes a separation membrane for separating the mixture gas (containing fuel vapor) from the canister into an air-rich component (gas) and a fuel vapor-rich component (gas). The fuel vapor-rich component discharged from the membrane separation module is fed to a condensing means for liquefying fuel vapor in the fuel vapor-rich component. The thus liquefied fuel vapor is recovered to the fuel tank.
However, in such a conventional fuel vapor treatment system provided with the membrane separation module, a semiconductor element using Peltier effect is employed as the condensing means. This not only makes the fuel vapor treatment system expensive but also increases electric energy consumption amount thereby increasing load to a battery.
It is an object of the present invention to provide an improved fuel vapor treatment system which can effectively overcome drawbacks encountered in conventional fuel vapor treatment systems.
Another object of the present invention is to provide an improved fuel vapor treatment system which can effectively liquefy and recover fuel vapor generated in a fuel tank, at a relatively low cost without using a conventional fuel vapor condensing means.
A further object of the present invention is to provide an improved fuel vapor treatment system including a membrane separation module, in which fuel vapor in a fuel vapor-rich component can be effectively cooled and liquefied under endothermic phenomena occurred in a fuel vapor adsorbing material during desorption of fuel vapor from the fuel vapor adsorbing material.
According to the present invention, a fuel vapor treatment system comprises a canister connected to a fuel tank and containing a fuel vapor adsorbing material which generates endothermic energy during desorption of fuel vapor. A membrane separation module is provided to be connected to the canister and including a separation membrane for separating a mixture gas purged from the canister into an air-rich component and a fuel vapor-rich component. Additionally, a condenser is provided to be connected to the membrane separation module to be supplied with the fuel vapor-rich component from the membrane separation module. The condenser is housed in the canister and adapted to cool and liquefy fuel vapor in the fuel vapor-rich component to obtain liquefied fuel by the endothermic energy generated in the canister, the liquefied fuel being recovered.